The Implications of Handling Qualities in Civil Helicopter
Accidents Involving Hover and Low Speed Flight
Daniel C. Dugan
Deputy Director
National Rotorcraft Technology Center
Moffett Field, CA
CDR Kevin J. Delamer, USN
Navy Liaison Officer
National Rotorcraft Technology Center
Moffett Field, CA
Because of increasing accident rates in Army helicopters in hover and low speed flight, a
study was made in 1999 of accidents which could be attributed to inadequate stability
augmentation. A study of civil helicopter accidents from 1993-2004, was then undertaken
to pursue the issue of poor handling qualities in helicopters which, in almost all cases,
had no stability augmentation. The vast majority of the mishaps studied occurred during
daylight in visual meteorological condition, reducing the impact of degraded visual
environments (DVE) on the results. Based on the Cooper-Harper Rating Scale, the
handling qualities of many of the helicopters studied could be described as having very
objectionable to major deficiencies. These costly deficiencies have resulted in
unnecessary loss of life, injury, and high dollar damage. Low cost and light weight
augmentation systems for helicopters have been developed in the past and are still being
investigated. The potential for a significant reduction in the accident rate exists.
Concerned by a trend of increasing
accident rates, Key published the results of
a survey of accidents that occurred during
hover and low speed flight involving four
types of Army helicopters (Ref.1). The
deficiencies in the accidents analyzed in
these pilot error mishaps were of primary
Presented at the American Helicopter Society 2005
International Helicopter Safety Symposium
Montreal, Canada, September 26-29, 2005.
Copyright © 2005 by the American Helicopter
Society International, Inc. All rights reserved
interest. The helicopters studied spanned
the range from the OH-58D Kiowa, a small
reconnaissance helicopter, the large cargo
transport CH-47, and included the AH-64
Apache attack helicopter and the UH-60
Blackhawk. All of these aircraft were
equipped with a rate command stability
augmentation system (SAS) which
provides pitch, roll, and yaw rate damping
inputs, of small magnitude at relatively
high frequency, to the control system.
These helicopters had been flight tested and
their handling qualities were assessed as
marginal. It was Key’s contention that the
modification of the rate command systems
to provide attitude command response,
would significantly reduce the accident
rates. With an attitude command system,
the control augmentation generally has
increased authority and attitude changes are
proportional to stick displacement.
Motivated by these results and conclusions,
a study of civil helicopter accidents was
undertaken to pursue the issue of handling
qualities and the role that they might play
in causing or contributing to mishaps.
National Transportation Safety Board
(NTSB) accident summaries were studied
and analyzed for the period 1993 -2004
(Ref. 2). Only hover, hover taxi, and low
speed flight summaries were considered, as
these represented a significant but bounded
portion of the total number of mishaps (Fig.
1). In addition to
24.3 %
Hover and
Low Airspeed
Data extracted from NTSB Aviation Accident Database, accessed
At http://www. [25 June 2005]
Hover and Low Airspeed Helicopter Mishaps
(as a percentage of all helicopter mishaps)
Fig. 1
limiting the scope of the inquiry to a
manageable level, this data set examined a
unique aspect of rotorcraft operations
which accounted for a significant portion of
the civil rotorcraft mishaps. It also aligned
the study with the results of the earlier Key
study of military accidents. The accidents
that were analyzed in this current study
occurred, in all but a few cases, on
helicopters that had no stability and control
augmentation and furthermore, with a few
exceptions, they did not occur at night or in
Referencing the Cooper-Harper Handling
Qualities Rating Scale (Ref. 3), it is the
authors’ opinion that the handling qualities
of a large number of the helicopters
involved could be categorized as having
very objectionable to major deficiencies.
Furthermore, it implies that extensive pilot
compensation is required for adequate
performance and, in the worst cases,
considerable compensation is required to
maintain control of the helicopter under
adverse conditions.
With few exceptions, the accidents
reviewed resulted in substantial damage, or
in some cases, the destruction of the
helicopter. This is not surprising since any
time the main rotor or tail rotor of a
helicopter contacts the ground, substantial
damage will result. The NTSB does not
assign a dollar value to damage classified
as “Substantial,” and the definition is very
broad in scope. “Substantial damage means
damage or structural failure that adversely
affects the structural strength, performance,
or flight characteristics of the aircraft, and
that would normally require major repair or
replacement of the affected part.” Large
sums may be spent to repair damage from
accidents that do not meet the NTSB Part
830 definition of “substantial damage.”
A study of recent (1963-1997) helicopter
accidents in the U.S. (Ref. 4), attributed
1,114 accidents, or 13.2% of the accidents
studied, to loss of control. In the categories
of single engine piston and turbine
helicopters, these accidents resulted in 247
fatalities, 228 serious injuries, and 319
helicopters destroyed. These categories of
helicopters generally do not have stability
and control augmentation systems installed
or available. It would not be possible to
speculate, without a detailed study of each
event, that any particular number or
percentage of these accidents could have
been prevented by the incorporation of rate
or attitude command control augmentation
into the flight control systems. Many of the
occurrences studied in Ref. 4 undoubtedly
were also analyzed in this current study,
since the time periods overlapped for some
of the years. Isler and De Maio (Ref. 5)
noted that the chain of accident events for
personal and instructional missions in a
large number of helicopter accidents began
with loss of control. These accidents, for
the low cost category of helicopters,
accounted for 48.2% of the total accidents.
In 1998, a Final Report of the joint
Department of Defense, Federal Aviation
Agency, and the National Aeronautics and
Space Administration Helicopter Accident
Analysis Team was published (Ref. 6). It
was surprising to note throughout the
report, that there was no mention of poor
handling qualities and the resulting failure
of the pilot to maintain control of the
helicopter as a factor in any of the
accidents reviewed from NTSB data. The
FAA’s National Aviation Safety Data
Analysis Center (NASDAC) reports were
recommendations in the Safety Investments
section of the report (Ref. 6) to improve the
designs of control systems to enhance the
handling qualities of the helicopters. As a
result of the Accident Team’s report, a
workshop was held in July of 1998 in
response to the recommendations of that
body. Subsequently, a report entitled “Near
Term Gains in Rotorcraft Safety –
Strategies for Investment” was published in
February, 1999 (Ref. 7). Although statistics
were discussed which tied many mishaps to
“loss of control,” once again there were no
recommended actions to improve civil
rotorcraft safety by the improvement of
helicopter handling qualities. Emphasis was
placed on training, obstacle protection, risk
management, accident analyses, and even
the use of night vision goggles.
The assessment of the accident reports
reviewed has, by necessity, involved
qualitative and subjective judgments by the
authors. The obvious cases that did not
involve handling qualities, such as
attempting to take off to a hover with one
skid tied down, were easily eliminated from
consideration. There were also cases where
pilots ran out of available power, which
often resulted in a loss of main rotor RPM
and subsequently a loss of control authority
- especially in the yaw axis. Controlled
descents into the ground, in many cases,
also had to be discounted as did mechanical
failures that resulted in loss of control.
Others involving loss of tail rotor control
power or authority were more difficult. In
many cases, hovering downwind or in a
stiff crosswind, might have resulted in an
accident by a pilot with low experience;
however, the experienced pilot could work
his way through the condition without
losing control. A student or low time pilot
will, in many cases, over control the
helicopter and use all of the available
control authority. There were many cases
where precision was required while
hovering near obstacles. Often in these
cases, little or no position drift could be
tolerated. At times, depending on the winds
and terrain, a “ skids level” attitude was
imperative at touchdown to avoid the
potential for a rollover or fore/aft pitching
which could cause main rotor or tail rotor
contact with the terrain.
A total of 547 accidents were thoroughly
reviewed from the years 1993 – 2004.
Initially, the accidents reviewed involved
only those occurring in hovering or near
hovering flight. This review was expanded
to include many accidents which occurred
in low speed flight either during takeoffs,
approaches to landing, or low speed
maneuvering. Of the total, 126 or 23
percent could be attributed to loss of
control by the pilot - caused or aggravated
by inadequate or deficient handling
qualities. There were also ten accidents that
involved gyroplanes or gyrocopters and
these were discarded from the matrix. It
was noted that there were three fatalities,
one serious injury, and four minor injuries
attributed to these ten, single pilot /
occupant, accidents. Pilot experience was
typically low and it may be inferred that
flying this type of hybrid rotorcraft is
particularly challenging for inexperienced,
though adventurous, aviators.
Many manufacturers were represented in
the accident statistics as compared to the
four models studied by Key (Ref. 1).
Fifteen different manufacturers were listed
to include homebuilt or kit helicopters
consolidated as one category. There were
over thirty various models represented.
Mishap Summary Examples
A few summary examples of mishaps
improvement in helicopter safety and
reduction in accidents that could be
achieved if helicopter handling qualities
were improved by the addition of stability
and control augmentation systems. While
these cases involve egregious examples of
handling qualities deficiencies, they are
unfortunately all to representative of the
accidents studied:
Case 1: This flight involved a surplus UH1H helicopter operated by the Tennessee
Valley Authority in daylight under
moderate, downwind wind conditions (9
knots gusting to 19 reported at an airport 5
miles east of the mishap site). It had flown
for 1.2 hours before the pilot attempted to
come to an OGE hover over a power pole
for the attachment of an external load.
Commands were given to the pilot to “ease
the aircraft down and hold position; move
left and move right”. The helicopter drifted
to the left and forward. After this, a
command to back off and try again was
followed by repetitive “move left”
commands. The main rotors struck a
workman on the pole, the pole itself, and
the helicopter rolled right in a descent and
struck the ground. There were no engine or
flight control malfunctions and the pilots
were highly qualified. Four fatalities and
one minor injury resulted from this
accident. This was a precision task for an
external load hookup in an OGE hover in
an unaugmented helicopter. It is our
contention that an improved control
system, to include stability augmentation,
could have enabled the pilot to achieve the
high degree of precision required to hold a
position and complete the hookup.
Case 2: A single engine, turbine powered,
teetering rotor helicopter crashed in snow
covered terrain in daylight under VMC
conditions with 1 mile visibility. The pilot
was attempting to land next to a snow
gauging station and he flew past it. With
the snow cover and reduced visibility, the
pilot had poor visual references and
allowed the skids to contact the terrain.
Collective pitch was applied, the pilot
attempted to stabilize the helicopter in
hover, but it drifted forward and to the left.
The left skid contacted the terrain and the
helicopter rolled on its left side. Although
there were no injuries to the three on board,
the helicopter was destroyed. The whiteout
conditions encountered resulted in a classic
loss of control in a degraded visual
environment. An effective stability
augmentation system could have allowed
the pilot to maintain a level attitude, hold
position, and either land level, pull up for
another landing attempt, or abort the
Case 3: A single engine (piston) helicopter
was to be flown on a maintenance test
flight for track and balance of the main
rotor. The pilot suddenly applied collective
pitch for take off and the helicopter rose to
about four feet and began to yaw left and
right and then pitch forward and aft. The
pilot attempted to stabilize the helicopter,
but the main rotor blades struck the ground
to the front and the helicopter rolled on to
its right side. There were no injuries to the
pilot or mechanic, but the damage incurred
was substantial. In this case, abrupt and
excessive control application caused the
pilot to lose control of the unstable
helicopter in hover. Although the pilot’s
control manipulation and activity were
determined to be the cause of the accident,
perhaps he could have recovered if the
helicopter had been a more stable platform
through the incorporation of stability
Case 6: A high-time fixed-wing pilot with
limited helicopter experience, flying a
encountered an uncommanded rotation to
the right. He failed to reduce the engine
throttle to idle and impacted the ground
after multiple rotations. The probable
cause determined by the NTSB was a loss
of tail rotor effectiveness (LTE) coupled
with the pilot’s failure to initiate a timely
Case 4: On a dual instructional flight, a
Student Pilot (SP) lifted the single engine,
piston engine, two-place helicopter to a
three-foot hover. The SP then “over
controlled” the helicopter and caused it to
drift to the left in a descent. The left skid
contacted the ground and the helicopter
rolled on to its left side. There were no
injuries, but the helicopter sustained
substantial damage. From reading many
similar accounts, these Student Pilots could
use more stable helicopters that are more
forgiving of their inexperience.
Stability Augmentation
Case 5: In another single engine, piston
engine helicopter on a dual instructional
flight, the student pilot developed a high
rate of descent at slow speed. A rapid
application of collective resulted in an
uncommanded rotation to the right which
the instructor was unable to arrest. In the
course of the attempt to regain rotor speed
lost during that recovery, the aircraft
impacted the ground and sustained
significant damage.
The issues which were causal factors, and
which also illustrated deficiencies in
handling qualities, could be grouped into
four broad categories as follows:
Helicopters are inherently less stable than
airplanes. If an unaugmented helicopter in
hovering flight is disturbed from
equilibrium, its attitude continues to
diverge until corrective control inputs are
made by the pilot. Most airplanes, however,
are inherently stable and when they are
disturbed from equilibrium, restoring
moments are generated. Only so much can
be done with the fundamental handling
qualities of the basic helicopter and
manufacturers generally design the low end
helicopter to meet the minimum
requirements of FAR 27.
aerodynamic modifications to the airframe
to improve handling qualities, such as fins
or strakes, are often costly, add weight, and
can produce additional drag with the
resulting performance penalties. Many
helicopters in use today do not have to
meet even the minimum standards for the
same class of helicopter being built under
the current requirements.
In the late seventies a Helicopter
Association International (HAI), then the
Helicopter Association of America (HAA),
held a convention in Mission Valley,
California near San Diego. The convention
was located in a hotel adjacent to a golf
course and helicopters were allowed to land
and take off in a clear grassy area next to
the parking lot. It was apparent from
operations observed at this temporary
heliport that a particular Bell 206 was very
stable while hovering, landing, and taking
off near the other operating helicopters.
The pilot’s control of pitch and roll attitude
was precise with very little oscillation.
From an orientation flight in the helicopter,
it was determined that it was equipped with
a Sfena “Mini-Stab.” This package had
been optimized for the pitch and roll axes
and the yaw axis augmentation was still
being developed. For the hover and in
flight operations, the improvements in
handling qualities were immediately
apparent. The pilot workload was reduced
significantly and precision was improved
considerably. It was an impressive
accomplishment for this small helicopter
which might otherwise be described as
“squirrelly” by some operators for some
tasks – to include hover and hover taxi in a
crosswinds and low speed flight in
A call to the Bell 206 Product Support
division in Mirabel, Canada, revealed that a
stability augmentation package is not
offered as an option for the Bell 206 series
of helicopters. This stability augmentation
system is installed in the Navy TH-57C
helicopters used for advanced flight
training and was required to meet military
standards for certification for single-piloted
instrument flight. This package was
designed in the late seventies and early
eighties and today’s technology should
permit the design of an inexpensive,
lightweight stability augmentation system
for even the low end helicopters on the
market. Hydraulic servo technology, where
applicable, has also progressed to permit
this integration at a lower cost and weight.
More recently, an Attitude Command /
Attitude Hold augmentation system,
HeliSAS, has been developed and tested
(Ref. 8). The system weighs only 12
pounds and the projected cost is $30,000. It
was demonstrated on an R-44 test aircraft
and it was evaluated by pilots from the
Robinson factory - as well as a NASA test
pilot and one from the National Test Pilot
School. The comments received were “very
favorable.” According to the FAA, a low
cost SAS has never been certificated,
considerable work needs to be done to
establish standards, and current rules are
not adequate. If these are reasons for not
proceeding, then the FAA needs to move
ahead with the formulation of standards
and revise the current rules. Light weight,
low cost systems such as the one described,
have the potential to significantly reduce
accidents attributed to poor handling
A recent simulation study at Ames
Research Center (April-May 2001), began
the process to look at the lowest levels of
stability and control augmentation that
could be used to safely operate a “low end”
helicopter single pilot under Instrument
Flight Rules (IFR). It would follow that the
application of even minimum levels of
augmentation for instrument flight to this
category of helicopter could make a
significant reduction in accidents in hover
and low speed flight under Visual
Meteorological Conditions (VMC).
Attitude or even rate stabilization can
reduce the possibility of a PIO developing
by providing a more crisp response to
control inputs about all axes and by
providing the damping that permits smooth
control of the helicopter’s attitude and
One particular type of operation that
requires precise positioning by the pilot is
the external load or sling load mission. It
involves pick-up and placement of loads
suspended beneath the helicopter. Often
they are conducted in close proximity to
obstacles and they require a high degree of
precision and demand a high workload of
the pilot. In this survey, 85 or 15 % of the
accidents occurred during external load
operations, far exceeding the FAA
determined helicopter utilization rate of 6%
for this mission area (Ref. 9), as shown in
Figure 2. Although most of these accidents
could not be attributed to poor handling
qualities, pilots involved in these
operations need a stable platform for their
precision task. Only six of the mishap
aircraft in external load operations were
equipped with an
Percentage of Total
position. Overshoot and over controlling
are reduced, if not eliminated, even for the
novice pilot. While manufacturers and
operators strive to hold down the
manufacturing, acquisition, and operating
costs of the lower end helicopters, perhaps
it would be appropriate for operators to reexamine the costs of these “loss of control”
accidents. As previously stated in this
review of hover, near hover, and low speed
accidents, it was a rare exception to find
other than substantial damage or
destruction of the helicopter. Substantial
Damage equals Big Money which the low
profit margin operator cannot afford to
sustain for long before his business is
threatened. Wouldn’t it be a better course
to invest additional dollars up front in the
acquisition of at least a minimal stability
and control augmentation system? The
helicopter could then be flown more safely
by pilots at all experience levels with more
precision and less pilot fatigue.
Flight Hours
External Load Operations
Fig. 2
attitude command system. An attitude
command system would have been a
significant improvement for the twentyone helicopters involved that did have
stability augmentation (rate command).
Any augmentation would be a step in the
right direction for the helicopters involved
in the remaining accidents. It was sobering
to calculate the toll from these external load
accidents as a matter of information. There
were 38 fatalities, 26 serious injuries, and
20 minor injuries. Twenty-six helicopters
were destroyed and all of the others, with
one exception, incurred substantial damage.
It follows that this operation can be
classified as hazardous regardless of
handling qualities.
Precise handling
qualities are dictated by the tight tolerances
required for these tasks. This highlights the
benefits accrued by incorporating stability
augmentation in helicopters performing this
Directional Control
The FAA Advisory Circular, AC-90-95,
“Unanticipated Right Yaw in Helicopters,”
(Ref. 10) states that the loss of tail rotor
effectiveness (LTE) is a critical,
aerodynamic flight characteristic which can
result in an uncommanded right yaw rate
which will not “subside” (damp) of its own
accord. If not corrected, it can result in the
loss of aircraft control. The AC also states
that “there is a greater susceptibility for
LTE in right turns. This is especially true
during flight at low airspeed since the pilot
may not be able to stop rotation. The
helicopter will attempt to yaw to the right
and correct and timely response to this
uncommanded yaw is critical. The yaw is
usually correctable if additional left pedal
is applied immediately. If the response is
incorrect or slow, the yaw rate may rapidly
increase to a point where recovery is not
For the accidents surveyed, 82 or 15%
involved LTE (Figure 3). Recognizing the
conditions that can lead to LTE is very
important and the pilot’s situational
awareness is critical at all times. What
direction is the wind with respect to the
helicopter’s heading? How much tail rotor
control margin remains in terms of pedal
applied? How much collective pitch can be
applied before maximum available power is
commanded and main rotor and tail rotor
15 %
All Other
Anyone who has flown or spent much time
as a passenger in any of a long list of small,
unaugmented helicopters, is aware of their
directional instability. A typical maneuver,
which illustrates the “squirrel like”
response of the machine, is the crosswind
hover or hover taxi. The pilot must
constantly make high frequency pedal
inputs of varying size to keep the helicopter
heading in the desired direction. The tail
boom dances to the right and left as the
position is held or as the taxi proceeds. It is
a high workload task and precision is
lacking. With a capable yaw SCAS,
especially one designed for attitude
command, the pilot’s workload and
precision, and the passenger’s comfort, are
dramatically improved.
A less attractive alternative might entail
building in more directional control power
by improving the tail rotor design to
produce more thrust, increasing engine
power available to provide that thrust, and
making suitable aerodynamic changes to
the fuselage and tail boom designs to
improve the yaw stability of the helicopter.
All of these changes, where applicable and
practical, could result in significant weight,
cost, and performance penalties.
Teetering Rotors
LTE Mishaps
(as a percentage of all mishaps studied)
Fig. 3
RPM start to decay? For the student or low
time helicopter pilot, these in flight
judgments are often gained only through
experience - which can come at a high
price in too many cases. The Army and Air
Force versions of the H-21 tandem rotor
helicopter (unaugmented), used for a few
years in the early stages of the Vietnam
conflict, always acted as though it wanted
to swap ends. Accordingly, the pilot
workload in the yaw axis was very high.
In a significant number of the accidents
incorporated a teetering rotor system. Of
547 accidents, 308 or 56 percent of the
helicopters had teetering rotor systems
Number of Mishaps
(Figure 4). The numbers may, on the
Sem i-rigid
Mishaps by Type Rotor System
Fig. 4
face of the data, be misleading. Teetering
rotor systems are incorporated in
helicopters that have been produced in
large numbers for many years. Two
manufacturers today are very successful in
supplying this teetering rotor system design
to the world market. One series of
helicopters is relatively inexpensive
(approximately $175,000 and up), powered
by a piston engine, and is frequently
utilized in pilot training. However, flight
training by its nature accounts for a
disproportionate number of mishaps. By
contrast, the rigid or hingeless rotor
systems tend to be incorporated in more
sophisticated aircraft, employed in specific
missions, such as medical transport, and
flown by more experienced pilots.
The semi-rigid or teetering type of rotor
accomplishes blade flapping by, as the
name describes, a teetering motion of the
two bladed rotor system. This gives the
helicopter a balanced lift distribution
between the advancing and retreating
blades and prevents a rolling moment from
developing. The teetering system differs
from the articulated system which allows
individual blades to flap, and the hingeless
rotor systems which accomplish flapping
by bending of the individual rotor blades to
change their angles of attack as the blades
advance and retreat. The teetering rotor in
particular, has a significant, built in phase
lag from the pilot’s control input to the
main rotor until the pitching or rolling
moment is generated. An absence of rate
damping only aggravates the condition.
This alone can cause a student pilot to put
in a larger control input than is required
which eventually results in a larger roll or
pitch rate than desired, which can
ultimately lead to a pilot induced
oscillation (PIO). Without an instructor
there to damp the oscillation, a roll over
can result if the main rotor or landing gear
should contact the surface.
The teetering rotor is susceptible to
conditions which can result in the rotor hub
contacting the mast. This is known as mast
bumping and unless it is just a very light
tap, the result can be a sheared mast in
flight which is usually fatal to all on board.
Unless, of course, the helicopter is at a very
low altitude and airspeed (hover IGE, for
instance). In the early design versions of
the OH-23 (UH-12), the hub had a circular
cutout for the mast. It wasn’t long before a
few fatal accidents took place and
investigation proved that mast bumping
occurred when excessive blade flapping
angles allowed hub to mast contact. The fix
was quite simple. The hub cutout was
elongated under the blades to allow
additional degrees of flapping and the
serious design flaw was resolved.
In this survey, only three of the hover or
low speed accidents were attributed to mast
bumping with subsequent shearing and
separation of the main rotor mast. These
accidents did result in four fatalities,
however. A single engine helicopter
accident in August, 2000 near Watsonville,
CA, which resulted in two fatalities, was
attributed to mast bumping by an operator,
demonstration team leader, and dealer. The
NTSB report also stated that the probable
cause was mast bumping. Although the
event occurred in cruise flight, and not
hover or low speed, the teetering rotor
design is susceptible to this type of accident
from excessive, uncoordinated, and abrupt
cyclic control inputs.
For all of these disadvantages, the teetering
rotor helicopter has one distinct advantage
over many helicopters that use an
articulated rotor system with flapping or
“droop” stops. That is the ability to start or
stop the rotor in much higher winds and
turbulence – with or without a rotor brake.
In addition, the design of the teetering rotor
system’s hub is not complex and it is quite
rugged and reliable with low maintenance
requirements. It does not have the
susceptibility to ground resonance which
designs can encounter. Blade attachment
failures for this simple design, with the
resulting catastrophic loss of a blade, are
rare, as compared to articulated rotor
designs. These features make it a choice for
many low end helicopters and also for the
homebuilt market.
Instrument Flight:
The first civil helicopter certificated for
flight in Instrument Meteorological
Conditions (IMC) in the United States was
the Bell 212. In order to achieve the level
of handling qualities imposed by the
Federal Aviation Agency, an expensive
package of instrumentation and stability
augmentation improvements had to be
incorporated in this twin engine helicopter.
Since that time, many high end helicopters
have been certificated for IFR operation to
include single pilot operation. Contrast that
with the long time operation of the
venerable UH-1 “Huey” by the Army under
Instrument Flight Rules (IFR) flying the
airways and making approaches in IMC to
both military and civil airports. This was
done in a helicopter whose total stability
augmentation consisted of the stabilizer bar
mounted above and 90 degrees to the main
rotor. It produced a slight damping of the
main rotor displacements by its gyroscopic
and inertial effect. Flying under IMC,
especially in turbulence, was a high
workload task in these military helicopters
and it required a high degree of training
and proficiency. For the civil version of
these helicopters (Bell 204, 205), flight
under instrument conditions is prohibited.
During the Vietnam War, the demand for
helicopter pilots was so great that the
practice of fully qualifying Army pilots for
instrument flight was discontinued. The
pipeline for pilots had to be filled as attrition
and rotation to other assignments after the
one year tour of duty, depleted the
“inventory” of pilots. As a stopgap measure,
a few hours of instrument training were put
in the syllabus and the pilots were awarded
“Tactical Instrument Tickets.” This minimal
training was tailored to permit the pilot to
get himself out of encounters with
inadvertent IMC or, in other words, perform
the 180 degree turn to fly back to visual
conditions. This did not always work and
many young pilots were not proficient. Add
the additional complication of night
conditions and the stage was set for many
non-combat deaths which occurred with
some regularity. Many days and nights were
IMC, especially during the long rainy
seasons, but the war had to go on and
missions had to be flown. Many helicopters,
such as the OH-6A, OH-58A, and the AH1G, were not designated for instrument
flight even by fully qualified pilots. The
entire fleet of Army helicopters at that time
would not meet FAA standards for
instrument flight. The handling qualities
were generally inadequate, most had no
stability augmentation, and if so equipped,
rate command was the system installed.
There have been attempts to “fix” poor
handling qualities of some helicopters by
the addition of avionics such as Flight
Directors. While these additions can reduce
the workload of the pilot and permit greater
precision during the instrument approach
task, they cannot improve the fundamental
flying qualities of a helicopter or any
aircraft. They are very effectively used in
combination with stability augmentation to
provide the pilot with the precision
required and a manageable workload. The
FAA imposes standards for handling
qualities as defined in the Federal Aviation
Regulation (FAR) Part 27; however, these
require only minimal standards. Military
helicopters must meet the requirements of
ADS-33D-PRF which are more stringent
than those of the old MIL-H-8501. Even
so, some military helicopters have had
extremely poor handling qualities in the
past and many of these helicopters are still
in military or civilian service today.
Very few of the accidents reviewed
occurred at night. Night conditions can
often be likened to instrument conditions
on a cloudy or moonless night where there
is no visible horizon. It can be speculated
that similar conditions can result in many
accidents due to spatial disorientation.
Many helicopters in use today do not have
the minimum instrumentation, much less
the stabilization, to be flown under real or
quasi-instrument conditions, even in
emergencies. The lack of any rate or
attitude stabilization in the design of
helicopters was continued for decades in
many helicopters, both military and civil,
and continues to this day.
It can be inferred that a significant
reduction in accidents, injuries, and
property damage could be achieved by the
integration of stability augmentation
systems into the control systems of the
lower priced helicopters.
Where this
investment has already been made in the
higher priced machines, the benefits of a
more sophisticated augmentation system
(Attitude Command) could be substantial in
terms of helicopter safety.
From the analysis made of the accidents
occurring in hover or low speed flight for
this current study, it can be inferred that a
documented accidents could have been
prevented if the mishap helicopters had
improved handling qualities.
From the accidents reviewed, and the other
statistics on civil helicopter accidents
attributed to loss of control, it is puzzling
why poor handling qualities have not been
pinpointed as causes or factors in the
accidents. Improvements in handling
qualities were not even recommended,
within the scope of this research, as a
means or investment in safety to reduce the
frequency of such accidents.
incorporating a low cost, light weight
stability augmentation system should be
explored by the helicopter airframe
manufacturers. Today’s technology may
provide the means to accomplish a goal of
significantly improving the handling
qualities of their helicopters. Where a
hydraulic system is not practical for
inclusion in the design, the technology
exists to provide the secondary or
automatic flight control system functions
with small electrical actuators. A reduction
in the accident rates will surely follow.
When the other safety investment strategies
are implemented, the goal of reducing the
accident rate by as much as 50% may not
be “pie in the sky.”
In 1966, NASA published the results of
handling qualities evaluations of seven
General Aviation aircraft manufactured in
the United States (Ref. 11). This
investigation revealed that the handling
qualities of the class of general aviation
aircraft, although generally satisfactory for
flight under Visual Meteorological
Conditions (VMC) and instrument flight in
smooth air, were degraded when
atmospheric turbulence was encountered.
designations and manufacturers were not
stated, the illustrations of the various types
of light airplanes evaluated effectively
identified the selected models.
It is recommended that representative
classes of light, piston engine and turbine
powered helicopters be similarly evaluated
to assess their handling qualities and to
document the deficiencies. In our opinion,
strong recommendations
forthcoming to improve the handling
qualities of this class of helicopter.
Appendix A
Comments on Adjunct Safety Issues
Post Crash Fires: Many helicopters
involved in survivable accidents have
experienced post crash fires that have
caused thermal injuries and fatalities to
crew and passengers. The Army pioneered
the development of crashworthy fuel
systems based on the incidence of post
crash fires in their fleet of helicopters and
the resulting injuries and deaths to crews
and passengers. An early historical
example involved the OH-13 helicopter.
Until the OH-13H was introduced, the
incidence of post crash fires in hover
accidents was nominal for this particular
helicopter. The “H” Model configuration
had two “saddle” fuel tanks mounted high
behind the cockpit bubble. As operational
time was accumulated on this new model,
the incidence of post crash fires and
thermal injuries increased dramatically in
fully survivable accidents. A study, which
included the use of a tethered helicopter
deliberately crashed from hover, revealed
that components of the rotating swashplate
(swashplate driver) struck the tanks and
slashed them open. The resulting fine fuel
spray was easily ignited. As a quick fix for
this problem, the Army devised a system to
wrap the tanks with a tough nylon fabric
impregnated with a resin to seal it to the
tanks after wrapping. The objective was to
provide some tear resistance to the tanks
and reduce the loss of fuel in the event of
swashplate contact. Many of these
helicopters are still flying today, without
any added protection, as Bell 47 models.
The expansion of Army Aviation during
the years of the United States’ involvement
in the Republic of Vietnam led the Army to
develop the Crashworthy Fuel System for
its fleet of UH-1 and other helicopters. The
design was simple, but it prevented many
post crash fires and the injuries and
fatalities which, in many cases, would have
resulted. The fuel bladders were
constructed of a heavier, tear resistant
rubberized fabric and break away fuel
fittings were incorporated in the fuel lines
and hoses. These fittings would break loose
on impact and seal the lines and tanks to
contain the fuel. On the downside, the
thicker bladders weighed more and reduced
the fuel capacity of the aircraft slightly. For
example, a UH-1H (Bell 205) original tank
held 220 gallons of jet fuel. After the
modified tanks were installed, fuel capacity
was 211 gallons or a 4% reduction. This
was not too great a price to pay for the
significant reduction in thermal injuries and
incorporation of this technology into this
large fleet of Army helicopters.
This technology, applied to the civil
variants of many Army helicopters, could
extend these safety benefits to larger
segments of the private and corporate
sectors. Amendments to FAR Parts 27 and
29 require that a crashworthy fuel system
be included in the design of any helicopter
for which a new type certificate has been
applied. It does not, however, apply to
helicopters certificated prior to the effective
date of the amendments. These helicopters,
which number in the thousands, will not
afford the crews and passengers the vital
protection from, and the reduced likelihood
of, post crash fires. Retrofit kits could be
designed and marketed for the civil sector.
The application of this technology should
also be considered for retrofit in the huge
fleet of General Aviation fixed wing
aircraft. It is true that it would involve
considerable expense and result in a small
reduction in fuel capacity. This should be
factored against the many lives that would
be saved in otherwise survivable accidents.
The disfiguring and incapacitating burns
that result in the survivable “crash and
burn” accidents should also be considered
when tallying up the true costs of the
modifications versus the human misery that
could be prevented.
The occurrences of post crash fires for the
hover and low speed cases studied were not
excessive, but the Army’s experience, prior
to the installation of the crashworthy
systems, was sobering.
Teetering Rotor Systems: As noted in the
main body of the report in the Results
section, the military services had many
fatal accidents attributed to mast bumping
in the UH-1 series of helicopters that were
employed by the thousands. Although a
significant number of mast bumping
accidents were not noted in this report, it
was considered important to describe the
conditions conducive to mast bumping for
those not familiar with this usually fatal
The problem was accentuated and
aggravated when the Army began to use
nap-of-the-earth and terrain following
tactics to avoid the shoulder fired missile
threat in the Republic of Vietnam. For
many years in Vietnam, the helicopters
flew at approximately 1500 feet above the
terrain to avoid much of the effective small
arms fire (7.62mm). With the introduction
of the Russian Strela heat seeking missiles
into the combat zone, the tactics changed
and put the helicopters “down on the deck.”
Mast bumping accidents began to occur
more frequently and it was learned that
they were due to a combination of
circumstances. When the pilot pushed the
nose of the helicopter down rapidly to stay
near or get close to the terrain, such as
flying over a ridge, the helicopter rotor
experienced less than 1g conditions. This
caused large rotor flapping angles. When
aggravated by a combination of abrupt
control inputs, aft center of gravity, or
sideslip, a fatal mast bump could occur.
Immediate training remedies were used to
make pilots aware of the conditions that
caused mast bumping and how to avoid
them. In addition, a thick walled mast was
developed to replace all of the rotor masts
in the entire fleet of UH-1 helicopters. The
intent was to be able to sustain a light mast
bump without shearing. Hub springs were
also incorporated in the design of these
teetering rotor helicopters to aid in
controlling the flapping of the main rotor.
While the lessons of this conflict only
bear peripherally on the discussion at hand,
they do begin to apply in the low airspeed
regime and would likely find even greater
significance in extending this work to an
analysis for the forward flight regime.
Appendix B
About the Authors
Daniel Dugan has Airline Transport Pilot
ratings for rotorcraft and for airplanes and
has flown over 3000 hours in helicopters
and over 4700 hours in airplanes. The
helicopters flown included many without
stability and control augmentation to
include the OH-13 series (Bell 47), OH-23
series (Hiller UH-12), UH-1 series (Bell
204, 205), H-21, OH-6A (Hughes 369),
Bell 206, OH-58, and Robinson R-22.
Many hours were also logged in augmented
helicopters to include the CH-47A and B,
AH-1, SH-34, SH-3, and CH-54. Both
piston type and turbine powered helicopters
were represented. He was a Master Army
Aviator and later a NASA research pilot.
He is a graduate of the United States
Military Academy, the Naval Test Pilot
School and he earned a Master’s Degree in
Aerospace Engineering from Virginia
Commander Kevin Delamer is a Naval
Aviator with 18 years experience as a
helicopter pilot and is a graduate of the
U.S. Naval Academy, the Naval War
College and the U. S. Naval Test Pilot
School. He has logged over 3000 hours in
32 type / model / series helicopters,
primarily in H-3 and H-60 series aircraft.
He is currently assigned to the National
Rotorcraft Technology Center and flies
with the Army Aeroflight Dynamics
Directorate at NASA Ames Research
6. “Final Report of the Helicopter
Accident Analysis Team,” DOD, FAA,
and NASA, July, 1998.
1. Key, David L. “Analysis of Army
Helicopter Pilot Error Mishap Data and
Implications For Handling
Qualities.” Paper presented at the
Twenty- Fifth European Rotorcraft
Forum, September 14-16, 1999.
7. “Near Term Gains in Rotorcraft Safety
Workshop sponsored by NASA Ames
Research Center, February, 1999.
2. NTSB Helicopter Accident Summaries,
8. Hoh, Roger. “Attitude Command
Attitude Hold Augmentation as a
Means to Enhance Safety for Light
Helicopters.” Presentation, April 2,
3. Cooper, G.E., Harper, R.P. Jr. “The
Use of Pilot Ratings in The Evaluation
of Aircraft Handling Qualities,” NASA
TN D-5153, April, 1969.
4. Harris, Franklin D., Kasper, Eugene F.,
Iseler, Laura E. “U.S. Civil Rotorcraft
Accidents, 1963 Through 1997,”
USAAMCON-TR-A-006, December,
5. Iseler, Laura E., De Maio, Joe.
“Analysis of US Civil Rotorcraft
Accidents from 1990 to 1996 and
Implications for a Safety Program.”
Helicopter Society 57th Annual Forum,
May 9-11, 2001.
9. FAA Aerospace Forecasts, Fiscal Years
2004-2015, U.S. Department of
Administration, Office of Aviation
Policy & Plans, March 2005
10. FAA Advisory Circular AC 90-95,
Helicopters,” February, 1995.
11. Barber, Marvin R., Jones, Charles K.,
Sisk, Thomas R., and Haise, Fred W.
“An Evaluation of the Handling
Qualities of Seven General Aviation
Aircraft,” NASA Technical Note TN
D-3726, November, 1966.